In the wake of the Oklahoma tornado, Nick Collins and Richard Gray, the
Telegraph's science correspondents, explain what they are and why the region
known as Tornado Alley is so susceptible

A tornado is a column of air which rotates at extremely high speed and stretches between a thunderstorm in the air, and the ground below. With wind speeds reaching above 250mph in severe cases, they are capable of causing serious damage.

What causes a tornado?

Tornadoes form when warm, moist air collides with cold, dry air. The warm air rises through the colder air creating an updraft. The twisting effect is triggered shortly before a thunderstorm develops as the wind changes direction and becomes faster at greater heights.

As more warm air is drawn in by the thunderstorm as it moves, the rotation speed increases while cool air fed by the jet stream high up in the atmosphere adds yet more energy. Water droplets from the moist air in the rotating air form a funnel cloud, which grows and descends downwards until it touches the ground, becoming a tornado.

The most violent tornadoes come from supercells, large thunderstorms that have winds already in rotation. About one in a thousand storms becomes a supercell, and one in five or six supercells spawns off a tornado.

What are the warning signs?

Tornadoes are often preceded by dark, greenish sky, large hailstones and thick cloud. They are also extremely loud, giving off a roar similar to a freight train. Some tornadoes are easily visible once they begin to pick up dust and debris, but others are hidden by rain or cloud. Forecasters can often predict severe storms well in advance but can only detect a tornado forming at about 15 minutes notice.

Why are they so destructive?

Tornadoes are much smaller and briefer than hurricanes, generally covering only a few kilometres and lasting no more than an hour. But wind speeds in severe tornadoes are much higher than in tropical storms, meaning the damage they cause is more localised but often more severe.

Why is Oklahoma so prone to tornadoes?

The area hit by the latest disaster is in an area known as "Tornado Alley", a region of the US Great Plains which experiences more tornadoes than anywhere in the world. Oklahoma alone sees 62 violent tornadoes a year.

During the spring in this region, the set of conditions which spawn a tornado become much more likely to occur. Moist, warm air is swept northward from the Gulf of Mexico by fast-moving currents and meets dry air drifting in from the Rocky Mountains. This clash can trigger "supercell" thunderstorms, a type of storm with a violent, twisting updraught which are a common cause of tornadoes.

How common are tornadoes?

There are on average around two to four tornadoes each day on the Great Plains of the US in May. Most do little damage, however, as they tend to be small and do not occur in densely populated urban areas. Just two per cent of tornadoes here reach EF4 status, with winds above 165mph.

According to the UK Tornado and Storm Research Organisation (TORRO), the UK experiences 30-40 tornadoes per year, although these are all far weaker and shorter lived than their US equivalents.

In July 2005 a tornado hit Birmingham where damage to trees, houses and cars was widespread across an area to the south-east of the city centre. It caused an estimated £40 million of damage and injured 19 people.

How are tornadoes measured?

They are measured using the Enhanced Fujita scale (EF scale), which measures the strength of tornadoes. It is a new version of the original Fujita scale, introduced in 1971 by Tetsuya Theodore Fujita. It has six categories from zero to five which represents different levels of damage according to the types of buildings affected. This ranges from little damage to roof gone to walls collapsed to blown away. Officials use 28 “damage indicators” – which are various types of building, ranging from small outbuildings to high rises and hospital buildings.

Has Moore in Oklahoma been hit by tornadoes before?

According to Dr Andrew Barrett from the department of meteorology at the University of Reading, Moore, in Oklahoma City has been hit by significant tornadoes four times in the past 15 years – in 1999, 2003, 2010 and the most recent one on Monday. The City of Moore itself states that there is only about a 1-2 per cent chance of a tornado of any size striking the city on any particular day during the spring.

Are there storm shelters in Oklahoma?

Moore has no designated public storm shelters, nor does Oklahoma City itself. Edmond also has no public shelters, but Norman has two and Midwest City has three. Residents in Oklahoma are offered the opportunity to receive a federal grant as part of a safe room rebate program, which gives them $2,500 upon installation of a shelter. In Moore, the city was expecting to apply for around $2 million to assist 800 home owners to build shelters. However, the programme was placed "on hold" as they waited for "wrinkles" to be ironed out of the grant application process.

What preparations are made for tornado season?

Oklahoma City has 182 tornado warning sirens located across its counties and these are activated once the National Weather Service issues a tornado warning. Residents are advised to go to their basement or storm cellar if they have one. If there is no basement they should go to the most interior hallway or room without windows. People living in mobile homes are advised to evacuate their property and seek shelter at a pre-planned location.

How tornado proof are homes in Oklahoma?

Oklahoma City government estimates that most homes in the area will provide adequate protection from 98 per cent of tornadoes. The city has no public buildings that are able to provide protection from an EF4 or EF5 tornado.